1. Field of the Invention
The present invention relates generally to printed circuit boards (PCBs), and more particularly to printed circuit boards having enhanced mounting pads thereon.
2. Disclosure Information
Surface-mount electronic components 30 are typically connected to printed circuit boards as illustrated in FIG. 1. First, a substrate 10 is provided which has a plurality of circuit traces 12 and mounting pads 20 thereon. The mounting pads 20 are arranged on the substrate 10 in matched relation with the terminations 32 of the electronic components 30 that are to be attached to the substrate. Next, depositions of solder paste 40 are placed atop each mounting pad 20, such as by screenprinting or dispensing, followed by placement of the components 30 atop the substrate such that each component termination 32 rests on the solder deposition 40 atop a respective mounting pad 20. Finally, the assembly is subjected to reflow soldering, laser soldering, or other processing so as to melt the solder depositions 40 and then allow them to cool to form solid solder joints 50 which mechanically and electrically connect each component termination 32 with its respective mounting pad 20.
The typical approach in depositing solder paste atop the mounting pads 20 is to deposit the paste in a 1:1 or less ratio with respect to each pad. For example, a rectangular 80.times.120-mil mounting pad may have a solder deposition thereatop that is a 50.times.90-mil rectangle generally centered atop the pad. However, the inventors of the present invention have discovered that it is advantageous in some applications to intentionally overprint the solder paste; that is, to deposit the solder in such a way that the deposition 40 extends outside the outer edges 22 of the mounting pad, as illustrated in FIG. 2. This is done, for example, to promote the formation of certain solder joint shapes during reflow.
Although overprinting provides certain advantages, it presents certain problems as well. One problem is that when using conventional, industry-standard design rules for circuit board construction, the overprinted solder deposition 40 extends partly atop the solder mask 14 which typically surrounds each mounting pad 20. (The solder mask 14 is a non-solder-wettable layer used on most PCBs for covering essentially all of the substrate surface, except for apertures 16 that are formed therein located about each mounting pad 20, as illustrated in FIGS. 1-2. The edges 18 of each aperture 16 are generally spaced outward from the corresponding mounting pad edges 22 by a certain distance P, called the "solder mask pullback"; for screenprinted solder mask, the pullback P is typically 10-20 mils, whereas for liquid photoimageable solder mask (LPISM) the pullback is typically 0-5 mils.) When the solder paste deposition 40 extends beyond the mounting pad edges 22 and onto the surrounding solder mask 14 as illustrated in FIGS. 3-4 (or, when no solder mask is used, onto the top surface of the surrounding bare substrate 10), there is a tendency to form solder balls 60 that are separate from the solder joint 50. The formation of solder balls 60 is undesirable for at least two reasons: (1) solder balls 60 take solder away from the solder joint 50, which often affects the joint 50 detrimentally, and (2) the solder balls 60 may roll across the substrate/solder mask and lodge between adjacent pads/terminations/joints, thereby causing unwanted solder bridges and short-circuits. These problems are presented regardless of whether the substrate 10 has a solder mask layer 14 thereon.
It would be desirable, therefore, to provide a way of allowing overprinting without the aforementioned drawbacks.